1. Field of the Invention
The present invention relates in general to gas detectors and, in particular, to a new and useful gas measuring and warning device which utilizes an indicator tube having material adapted to change color as the gas moves through the tube.
2. Description of the Prior Art
Indicator tubes are known which are filled with a powdery or granular chemical reagent and which generally serve the purpose of detecting gaseous components in gas mixtures which are directed through the tube by suction or pressure. During such a process, these tubes become progressively colored in the direction of the gas flow. The length of the colored zone is proportional to the amount of the gas component which had passed into the tube and to which the reagent is specifically responsive. If the quantity of the carrier gas directed through the tube by suction or pressure is known, the percentage of contamination of the carrier gas by this specific gas component can be determined from the length of the colored zone.
Such tubes are employed both for single tests and, in connection with a gas feed pump, for long-term monitoring. In either case, information is obtained on the amount of the gas to be measured that has passed into the tube. Particularly in long-term monitoring, the length of the colored zone read at the end of the monitoring period is a measure of the concentration of the gas to be measured, averaged over the entire period. In many instances, however, it is of importance to know in addition whether peaks or minima of concentration of the gas to be measured occurred some time during the monitoring period. This information can be used to instantaneously start an alarm if critical concentration values have been exceeded or not reached.
One prior art device for determining and registering certain components in the air or other gases makes it possible to determine the instantaneous concentration within shorter monitoring periods. In this device, indicator tubes accommodated in a magazine and already opened are sequentially brought into an air stream to be tested. The magazine is periodically and automatically indexed by a switching mechanism, such as a switch clock, so that a new indicator tube is always brought into the path of the air stream. The tube remains in this position until the air feeding device has passed the air through the tube under predetermined conditions.
This device is mechanically very complicated. The measured values are determined through observation and thus the reliability depends on the skill of the operator. A very disadvantageous aspect of this device is that an alarm cannot be started automatically (German Pat. No. 10 93 113).
Another prior art device for detecting gas concentrations, especially CO concentrations in the air, starts an optical and/or acoustic signal after a certain time, at a definite gas concentration indicated by the color change of the reacting layer. The device comprises a mechanically driven pump, a mount for an indicator tube at the suction side or pressure side of the pump, and a photoelectric scanner. The scanner is mounted for displacement along the indicator tube so as to scan a definite area of the indicator tube received in the mount. The intensity of the light beam from the photosensitive element of the scanner directed on the scanned area and reflected, which changes with the color of the reactive layer in the scanned area, varies the delivered current and thereby starts an optical and/or acoustic alarm. This measuring or warning device is based on the measurement of the total amount of foreign gas passing through the indicator tube and indicated by the length of the colored zone. The instantaneous concentration of the gas in the air to be tested cannot be determined. Just this quantity, however, is what is determinative. They physiologic effect of poisons depends not only on the absored amount but very strongly also on the concentration. In this respect, a measurement of the amount may even make no sense (German Offenlegungsshrift No. 15 98 021).
In still another prior art gas measuring and warning device, the monitored air is forced by pressure or suction through the indicator tube. The gas which is specific for the tube is measured, its concentration is determined and, upon exceeding certain limits, an alarm is started. To this end, the migration velocity v of the coloration front between the colored zone and the initial color is determined. This is done by means of light barriers, each comprising a light-emitting diode and a detector and light intensity of the sequentially interrogated light barriers is compared with the value recalled from a reference light barrier and converted into a yes-or-no signal. The monitoring is done in interrogation cycles and controlled by an inquiry circuit. In each cycle, a light emitting diode is sequentially triggered and the corresponding detector is interrogated. The obtained yes-signals are stored in a first counter, and the number of yes-signals from the following interrogation cycle are stored in a second counter. After each cycle resulting from the setting of the time interval, the two contents of the counters are compared with each other. The difference which is found is a measure for the velocity of migration v of the color front, and from this the gas concentration can be determined and utilized for the alarm.
In this known device, the accuracy of measurement, thus the determination of the speed of migration v of the color front, depends directly on the number of the light barriers per unit length. Therefore, to obtain a high accuracy of measurement, high costs of optical and electronic equipment must be taken into account (German Auslegungsshrift No. 26 28 790).